1. Field of the Invention
The invention relates to a method of microscoping an object with an interference microscope with extremely high resolution, by which method the object is illuminated through a lens by means of a light source, whereby the light beam of the light source is divided by a beam divider into at least two coherent part beams, of which at least one serves for illuminating the object, and the part beams, following illumination of the object and, if need be, of a reference object, are interferingly combined again and supplied to an interference figure detector screened in figure zones, for producing at least one interference figure, and based on the figure information of one or a plurality of interference figure(s), a phase image is calculated by means of a suitable algorithm, for obtaining a resolution beyond the diffraction limit, i.e., a location uncertainty .DELTA.x to be determined on the object smaller than one half wavelength of the light used.
Furthermore, the invention relates to an interference microscope, for example of the Linnik-type, with a light source for illuminating an object; with a beam divider for dividing the light beam emitted by the light source into at least two coherent part beams, of which at least one is intended for illuminating the object; and with an interference figure detector screened in figure zones, for example a pixeled CCD-line, for generating an interference figure of the interfering part beams, preferably for carrying out the aforementioned method.
2. The Prior Art
Techniques of phase-measurement interference microscopy are known, for example from the report by Catherine Creath "Phase-Measurement Interferometry Techniques", published in E. Wolf, Progress in Optics XXVI, Amsterdam 1988, pp 350-393.
In an interference microscope, it is generally possible to convert the intensity pattern--which is formed in the plane of the image by interference of the object wave and a coherent reference wave--into a phase image, namely by scanning or recording the image through a screened interference figure detector, and by further processing the obtained image or a number of images by a software algorithm. Various algorithms and their advantages or merits are reviewed in the report cited above.
Especially the four-step procedure, the so-called four-bucket-method addressed in the above-cited report in chapter 3.4, plays a role for the present invention, said method being explained in the following as background knowledge. This is done in view of the fact that according to the type of microscopy specified above, the objective is to achieve through interference microscopy a resolution beyond the diffraction limit, i.e., of a location uncertainty to be determined smaller than half the wavelength of the light used, a so-called super resolution.
The so-called four-bucket algorithm uses four intensity images in order to obtain a phase image. The intensity in a (two-beam) interference pattern can be described by: EQU l(x,y)=a.sup.2 (x,y) (1+.mu.(x,y) cos .phi.(x,y), (1)
whereby a(x,y) is the real amplitude, .mu.(x,y) is the band contrast (modulation depth), and .phi.(x,y) is the phase image to be obtained. This intensity distribution is scanned by the detector. For pixels (i,j) of a CCD-detector, the following is obtained thereby: EQU I(i,j)=a.sup.2 (i,j)(l+.mu.(i,j) cos .phi.(i,j)). (2)
Various types of microscopes can be used in this connection as interference microscopes, for example Mirau, Nomarski, etc. However, a microscope of the Linnik-type is preferred. Such microscopes are described, for example in H. Beyer and H. Riesenberg, "Handbuch der Mikroskopic" (Handbook of Microscopy), Berlin, 1988, chapter 6.2.
With a microscope of the Linnik-type (as it is shown in FIG. 1), a light beam is divided by means of a beam divider into an object beam and a reference beam, whereby the object beam is reflected in this case by the object, and the reference beam is reflected by a reference mirror. Subsequently both beams are interferingly supplied to the detector. The reference mirror is movable in the direction of the reference beam.
For the application of the aforementioned four-step method or the so-called four-bucket method, the reference mirror is moved with respect to its original position across each of the distances 1/4.lambda., -1/8.lambda., and 1/8.lambda., whereby .lambda. is the wavelength of the light used from the light source. From this, the following four images are obtained: EQU I.sub.1 =a.sup.2 (1+.mu.cos .phi.) (3) EQU I.sub.2 =a.sup.2 (1-.mu.cos .phi.) EQU I.sub.3 =a.sup.2 (1-.mu.sin .phi.) EQU I.sub.4 =a.sup.2 (1+.mu.sin .phi.).
The pixel coordinates were omitted for said four equations for reasons of convenience.
The equations (3) show that the following is applicable to each pixel: EQU tan .phi.=I.sub.4 -I.sub.3 (4) EQU I.sub.1 -I.sub.2.
Of course, .phi. is obtained between -.pi./2 and +.pi./2. A development method would be required for greater values. The specified algorithms are available in commercially available software. However, all phase-measurement algorithms as described in the cited paper by Creath suffer from the addressed problem of limitation for the value of .phi., in any case up to a certain degree.
The aforementioned four-bucket algorithm has an advantage and a drawback as compared to other algorithms as described in the paper cited above. The advantage consists in that said algorithm is relatively insensitive to detector non-linearities. The drawback consists in that the algorithm is relatively sensitive to errors in the /2-phase steps and thus with respect to the positioning of the reference mirror. This makes the commonly used digital interferometer systems less suitable for the desired high resolution (super-resolution). In practical application, they achieve an accuracy of (only) up to .lambda./100, and this only under particularly favorable circumstances, whereas an accuracy of .lambda./1000 is desired or required for refined experiments of super-resolution. The aforementioned four-bucket algorithm is nevertheless the best candidate for improving the accuracy in view of the desired accuracy.